1. Field of the Invention
This invention generally relates to integrated circuit (IC) ferroelectric thin film processes and, more particularly, to a doped PGO thin film, and corresponding fabrication processes.
2. Description of the Related Art
Ferroelectric-gate controlled devices, such as metal-ferroelectric-silicon field-effect transistor (MFS FET), were studied as early as the 1950s. In recent years, ferroelectric-gate controlled memory transistors have attracted much attention for high-density nonvolatile memory applications. Two kinds of device structures, Metal-Ferroelectric-Insulator-Silicon (MFIS) FET and Metal-Ferroelectric-Metal-Oxide-Silicon (MFMOS) FET, have been fabricated thus far. The ferroelectric Pb3Ge5O11 (PGO) thin films have a smaller polarization, a relatively large coercive field, and a lower dielectric constant. Therefore, PGO is a prime candidate for one-transistor memory applications. Extremely high c-axis oriented PGO thin films can be deposited on Pt and Ir metals, and working 1T-memory devices with PGO MFMOS memory cells have been fabricated. Even so, high quality 1T devices with good retention properties have been found to be difficult to fabricate. The problems result from the poor quality of the ferroelectric thin films, the leakage current of 1T devices, and the trapped charges in ferroelectric and oxide capacitors. These integration-induced damages are largely the result of PGO etching processes.
PGO is a ferroelectric material with low dielectric constant of about 50. This material is a low-symmetry ferroelectric material. Its spontaneous polarization exists only along one axis (c-axis) and has a value of 4–6 micro-Coulombs per square centimeter (μC/cm2). Conventionally, the Curie temperature of PGO is about 177° C.
Pure c-axis oriented PGO thin films have been successfully grow on Ir, Pt, Pt/Ir, and Ir—Ta—O conductive electrodes, as well as on ZrO2, HfO2, Zr—Hf—O, Zr—Al—O, Hf—Al—O insulators, using a spin on method. The resultant structures can be used for MFMOS and MFIS single transistor memory applications.
The Curie temperature of conventional PGO thin films is relatively low, causing thermal stability concerns. Once the working temperature exceeds 120° C., large leakage currents are typically observed. As a result of the low Curie temperature PGO film, the thermal stability of MFMOS and MFIS structures, formed from such a film, is impacted.
It has been reported that doping can modify the properties of ferroelectric thin films, but very little research has been performed on the subject of PGO thin film doping. This is because the ferroelectric Pb5Ge3O11 phase has a particularly narrow growth window.
It would be advantageous if the PGO thin film properties relating to the Curie temperature could be modified.
It would be advantageous if the Curie temperature properties of PGO thin films could be modified by doping the PGO film.
It would be advantageous if ferroelectric capacitors, suitable for use in 1T memory applications, could be fabricated using a PGO thin film having a higher Curie temperature.